Use FileCheck's --implicit-check-not='ThreadSanitizer' which increases
the strictness of our tests a bit.
CHECK: start CHECK-NOT: ThreadSanitizer CHECK: done
With --implicit-check-not='ThreadSanitizer' the above is turned into:
CHECK-NOT: ThreadSanitizer CHECK: start CHECK-NOT: ThreadSanitizer CHECK: done CHECK-NOT: ThreadSanitizer
Should this be removed now?